Internal bubble-suppression method and apparatus

ABSTRACT

The present invention comprises an apparatus and method for producing a high resolution acoustic signal from a single point source while achieving a commercially acceptable secondary bubble suppression. More specifically, the present invention comprises first and second explosive generators which sequentially create two explosions or one explosion and injection, within the body of water; said first explosion producing within the body of water a powerful acoustic pulse in an expanding cavity of very low pressure, said second explosion or injection establishing hydrostatic pressure within the cavity about the same time the volume of the cavity reaches its maximum volume, thereby substantially reducing the secondary pressure pulses.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Applicant's co-pendingapplication Ser. No. 703,302, now allowed as U.S. Pat. No. 4,735,281.The disclosure of the parent application, filed 2/20/85, is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to methods and apparatus which employhigh energy gas bubbles to generate underwater, sharp, impulsiveacoustic signals especially useful in marine seismic exploration. Theinvention relates more particularly to improve such methods andapparatus wherein secondary acoustic pulses or signals are suppressed.

2. Description of the Prior Art

Certain seismic sources such as explosives, air guns, gas exploders,etc., are purposely fired deep under water. It is well known that suchfiring creates a gas bubble or cavity and that the water acquiresoscillatory energy which generates acoustic pressure wavelets, eachconsisting of a desired "primary" acoustic pressure pulse having anamplitude P_(o), which is especially useful for most seismic explorationwork, and which is followed by an oscillating succession of undesired"secondary" (sometimes called "bubble") acoustic pulses of decreasingamplitude. In this specification, the words "bubble" and "cavity" willbe used interchangeably.

For example, an air gun explosively releases a high pressure gas bubblehaving an energy E_(a) into the water which creates a desired primarypressure pulse having a maximum amplitude P_(o). After the releasedhigh-pressure gas bubble impulsively contacts the surrounding water, itcontinues to expand as the water first accelerates outwardly and laterdecelerates until the cavity attains a maximum volume and comes to rest,at which time the pressure within the cavity is much less than thesurrounding hydrostatic pressure. At this point, the cavity has attaineda maximum volume V_(m).

When the expanding bubble reaches its maximum volume, there ispractically a vacuum inside the bubble, the kinetic energy of thesurrounding water is zero, and this water possesses maximum oscillatorypotential energy which, if not suppressed, will change into kineticenergy, back into potential energy, etc., for a duration of severalcycles, each having an oscillatory time period T.

The water gains maximum potential energy at 1/2 T, at which time thewater is ready to change course and rush inwardly to implode the gas inthe bubble. After one complete cycle, i.e., at time T, the bubble isrecompressed into a relatively small-diameter, high-pressure bubble. Thesurrounding water comes to an abrupt stop resulting in a first positiveacoustic secondary pulse having an amplitude P₂ which is mainlydependent upon the maximum kinetic energy acquired by theinwardly-moving water. The less kinetic energy acquired by the water,the smaller the amplitude P₂ will be.

Thus, the secondary pulse phenomenon occurs when the surrounding waterfirst violently implodes the bubble to a minimum diameter or volume, thekinetic energy is again zero and the potential energy is mainlycontained within the recompressed gas inside the bubble. This potentialenergy causes the bubble to again explode in its oscillatory scheme aspreviously described.

In this manner, the oscillatory energy stored in the water producesseveral successive secondary pulses of decreasing amplitude until aportion of the energy of oscillation becomes dissipated by naturalprocesses, such as turbulence, and the remaining portion is consumed toproduce the undesired secondary pulses.

The number of such bubble explosions (expansions) and implosions(contractions) may vary, but typically four to six significant secondarypulses can be expected after each primary pulse P_(o) which is generatedby the seismic source. Hence, a substantial portion of the acousticenergy released by the seismic source goes to waste because only aportion of the energy contained in the released gas is used to producethe desired primary seismic pulse P_(o), while the remaining andsubstantial portion of the energy becomes converted into harmfulsecondary seismic pulses having amplitudes P₂ which must be suppressed.

In seismic exploration, both the primary and secondary acoustic pulsesact as distinct acoustic disturbances which travel in the water in alldirections, penetrate the earth, strike one or more rock formations orreflectors, and then return into the body of water. The primary andsecondary pulses produce reflected seismic wavelets. But, since thesecondary pulses and their reflected waves occur at times when thereflected primary waves also return from the subterranean reflectors,the secondary pulses and their reflected wavelets interfere with thereflected primary waves. Further, because the reflected secondary wavesand the reflected primary waves are similar in shape, no practical wayhas yet been found for distinguishing between them.

In conducting marine seismic surveys, the reflected primary andsecondary seismic waves are sensed by detectors within a towed streamercable. The detectors faithfully transform the received acoustic seismicwaves into corresponding electric signals which are processed intoseismic traces that contain appreciable noise. This noise is due mostlyto the oscillatory secondary pulses which accompany each primary pulse.Under these noisy conditions, computations of the depths at which therock formations lie become very difficult and sometimes altogetherimpossible. This noise hinders the main object of the seismicexploration, which is, of course, to identify the various subterraneanformations from an interpretation of the seismogram sections produced bythe seismic survey.

The secondary-to-primary ratio amplitude P₂ /P_(o) is the benchmark bywhich all marine seismic sources are measured as to bubble suppression.An "ideal" source is said to be that source which has a ratio P₂ /P_(o)=0 for a frequency range from 0-125 Hz. Therefore, the extent to which aparticular seismic source approaches the ideal seismic source can bereadily measured by measuring its P₂ /P_(o) ratio.

An ideal seismic source produces a single, short, sharp acoustic impulsehaving sufficient energy and no secondary pulses. Sharp impulses areneeded to improve the definition of seismic reflections, becauseresolution is inversely proportional to the time-width of the impulse:the larger the time-width of the impulse, the less desirable it is.Fired near the water surface, a dynamite charge or other similarconcentrated explosion closely approximates the ideal seismic source,because the bubbles resulting from each explosion are vented immediatelyinto the atmosphere, hence there are no bubble implosions. If not firednear the water surface, explosive seismic sources will produce undesiredsecondary pulses, unless some form of implosion suppression is utilized.Explosive seismic sources include explosives, air guns, gasguns,expandable sleeve devices in which propane and oxygen are mixed to causeinternal combustion, etc. All of these share the common bubble problemfor which there has been no fully satisfactory solution, even thoughthere has been a long-felt need to find a mechanism to enhance thedesired primary pulse at the expense of the undesired secondary pulses.

In the absence of such a mechanism, many attempts have been made in thepast twenty-five years or more by the energy industry and their seismiccontractors to develop techniques for reducing the burden, financial andtechnological, imposed by the generation of the undesirable secondarypulses. These efforts have been directed toward attenuating theoscillatory secondary pulses and/or to reduce their ill effects. Fromthe initial introduction of marine seismic sources, there has been acontinuous need for effective and economical bubble suppression devices.That need and the various solutions offered to fill that need are welldescribed in the technical and patent literature.

One early mechanical technique attempt to prevent the secondary pulsesfrom traveling vertically downward towards the water bottom involved amethod whereby the gas bubble source was substantially enveloped in acontainer or cage having perforations, such that the expanding gasbubble would have to do work in order to force water through theperforations. The work performed by the expanding gas bubble dissipatedits internal energy, so that the ensuing secondary pulses would havereduced amplitudes. This technique has been used in a seismic sourcetrademarked FLEXOTIR. A serious limitation inherent in this techniquehas been that the desired primary pulses also become reduced in strengthbecause they can travel freely only through the available perforations.Also, the perforated cage becomes subjected to rapid deterioration, dueto the great stresses to which it becomes subjected when largedifferential pressures become exerted across its wall.

Various software programs have also been developed, for example, inconnection with the MAXIPULSE (trademark) seismic source, which utilizefast and powerful digital computer which produce seismograms from whichthe detected noisy seismic waves, caused by the deleterious bubbleeffects, have been removed so that the seismograms can be easierinterpreted by the geophysicists. However, running such programsrequires the use of expensive computer time and manpower, see e.g., U.S.Pat. No. 3,592,286.

Other prior art techniques have been based on air being injected intothe expanding bubble for shaping the secondary pulses. The knownapplications of the air injection technique have led prior art workersto very disappointing results and most of them were abandoned.

Due to the inefficiency or impracticability of known bubble suppressiontechniques, the seismic industry has also been obliged to employ a"tuned" array of seismic sources. Typically, these sources are air gunsof markedly different sizes. In theory when all such seismic sources aresituated in a tuned array, and then fired simultaneously, the amplitudeof the resulting primary pulse of the array will be equal to the sum ofthe amplitudes of the individual primary pulses generated by theindividual acoustic sources. Conversely, the amplitudes of the secondarypulses will theoretically be reduced because (1) they are not in phase,(2) they occur at different times, and (3) they have random frequencies.

Though the aforedescribed array technique has been the standard in thisart, this technique has presented serious drawbacks since the resultantseismic signature is only a composite of individual sources, each sourcelacking a narrow, sharp acoustic pulse as required. Also it has beenvery expensive to build such an array because it has required a largenumber of differently-sized air guns, as well as heavy and expensive aircompressors, to provide the appropriate volume of pressurized gasconsumed by the large number of airguns. Additionally, there is also aneed to maintain on boat a large inventory of spare parts to keep thedifferently-sized sources operational. The spare part problem is veryserious, because in many parts of the world they are not available andthey must be flown in from the home base. Many parts break down dailyand some weekly due to salt water, pollution, unsuspected debris, highpressure, etc.

SUMMARY OF THE INVENTION

The present invention addresses the above noted and other disadvantagesof prior art marine acoustic generators and their methods of use byproviding an economical apparatus and method of producing a highresolution acoustic signal from a single point source while achieving atleast commercially acceptable, secondary bubble suppression.

The present invention substantially reduces the pressure pulse generatedby an implosion of a cavity within a body of water by aborting theimplosion. The abortion of the implosion is produced by generatingwithin the cavity an injection before or at the instant this cavityattains its maximum volume V_(m). This injection must have sufficientenergy E_(b) to establish hydrostatic pressure P_(H) within this cavityclose to the time that the cavity becomes stationary. Preferably, thisinjection must establish hydrostatic pressure at the time the volume ofthe cavity first becomes stationary. Expressed otherwise, the presentmethod may be used to generate within a body of water an impulsiveacoustic signal by generating a first explosion within the body of waterto produce therein a powerful pressure pulse and a cavity of very lowpressure, and then generating an injection of gas within the cavity, soas to establish hydrostatic pressure P_(H) inside the cavity such thatthe volume of the cavity becomes stationary about the time the pressurewithin the cavity equals hydrostatic pressure.

The apparatus of the invention comprises first and second explosivegenerators which sequentially create two explosions or one explosion andan injection, within the body of water: a first explosion which produceswithin the body of water a powerful acoustic pulse and an expandingcavity of very low pressure, and a second explosion or injection whichestablishes hydrostatic pressure within the cavity about the same timethe volume of the cavity reaches about its maximum volume, therebysubstantially reducing the ensuing secondary pressure pulses.

The present invention has a number of advantages over the prior art.First, the apparatus and method of the present invention enablescomplete or at least commercially feasible secondary pulse suppressionwhile utilizing a single point source.

Yet another advantage of the present invention is the ease and economywith which secondary pulses may be "tuned out" of the acoustic signatureby the modification of a variety of operational parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an oscillating bubble in a body of water;

FIG. 2 is a pressure vs. time signature of FIG. 1;

FIG. 3 illustrates the behavior of bubble 1 when acted upon by bubble 2in accordance with the invention;

FIG. 4 is a pressure vs. time signature corresponding to FIG. 3;

FIGS. 5, 7, 8, 10-12 are sectional views of the present invention,showing six operating phases thereof;

FIG. 6 is a sectional view taken on line 6--6 of FIG. 5;

FIG. 9 is a sectional view taken on line 9--9 of FIG. 8.

FIGS. 7A, 8A, 10A-12A are pressure signatures obtained with theembodiment shown in FIGS. 7, 8, and 10-12 respectively;

FIG. 13 is a graphical representation illustrating the conservation ofenergy for an expanding bubble without any injection of gas.

FIG. 14 is a graphical representation illustrating the conservation ofenergy when an injection of gas is made.

FIG. 15 graphically represents the variation of the dimensionless ratioP'_(m) /P_(H).

FIG. 16 graphically represents the conditions for total bubblesuppression.

FIG. 17 graphically represents the conditions for total bubblesuppression when the injected energy E_(b) is slightly larger than E_(a)/k-1.

FIG. 18 graphically represents the variations of the new maximum volumeV'_(m) versus the injected energy for a total bubble suppression, andthe possible paths followed by an ideal injection.

FIG. 19 graphically represents the curve described by equation ##EQU1##

FIG. 20 graphically represents three types of injections with differentratios t₁ /T and Δt/T.

FIG. 21 graphically represents the curve θ/T as a function of the energyratio E_(b) /E_(a).

FIG. 22 graphically represents two methods for tuning θ/T to E_(b)/E_(a).

FIG. 23 graphically represents yet a third method to tune θ/T to theenergy ratio E_(b) /E_(a).

FIG. 24 graphically represents the variations of P₂ /P₀ versus t₁ /T.

FIG. 25 is a pressure signature according to the present invention andillustrating the bubble stretching.

FIG. 26 is a pressure signature according to the invention and using alarge energy ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENT

1. Generally

A general theory upon which the method and apparatus of the presentinvention is based is that generally descriptive and applicable to thelaw of thermodynamics and fluid dynamics. More particularly and as maybe seen in the accompanying illustrations, the present invention relatesto the periodic, underwater release of a compressed gas such as to forma series of sharp acoustic signals.

When a charge of highly pressurized gas having a pressure P_(a) isexplosively released at a sufficient depth within a body of water havinga hydrostatic pressure P_(H), a bubble is created such as to define aprimary pulse having a maximum amplitude P_(o). See FIG. 1 and 2. At theinstant the gas is released, the internal pressure P_(i) is quite largecompared to the surrounding hydrostatic pressure P_(H), and thereforethe bubble will radially expand, setting the water into an outwardmotion. At the instant the gas is released, this internal pressure P_(i)is a maximum, hence P_(i) =P_(a).

During this radial expansion, the internal pressure of the bubble P_(i)decreases until it renders an equilibrium value P_(H) and the wateracquires kinetic energy. However, this kinetic energy stored in thewater allows the bubble to expand beyond its equilibrium state--forwhich the internal pressure P_(i) =P_(H), and thus the bubble continuesto expand up to a maximum volumetric value V_(m).

When the bubble establishes this maximum volume V_(m) at time T/2 afterits initial release, its internal pressure P_(i) has fallen well belowhydrostatic pressure P_(H), and a near vacuum is established in thebubble. At such an instant, the kinetic energy of the surrounding wateris zero, but the potential energy of the water is at a maximum. Thebubble now begins to contract.

Because the hydrostatic pressure P_(H) is now larger than the internalpressure P_(i) of the bubble, the aforedescribed process begins toreverse. Now, the bubble begins to contract with a correspondingincrease in internal pressure. Upon this collapse, the surrounding wateracquires kinetic energy. When the bubble passes its static equilibriumstate, i.e., when the internal pressure P_(i) of the bubble is equal tothe hydrostatic pressure P_(H) of the water, the kinetic energy storedin the water is at a maximum, causing the bubble to collapse furtherwith a corresponding increase in internal pressure P_(i), to a pointsubstantially above the hydrostatic pressure P_(H).

At the end of the described cycle at a time T after the initial releaseof the pressurized gas the bubble is unable to collapse any further,thus abruptly halting the inward rush of the imploding water. Thisabrupt halt in the collapse of the bubble results in a first acousticsecondary pulse having an amplitude P₂ when measured at a distance rfrom the bubble's center.

At this point at a time T, the bubble is recompressed into a smallvolume with an internal pressure P_(i) far in excess of the surroundinghydrostatic pressure P_(H). As a consequence, the bubble again undergoesa radial expansion--reproducing the process with a period T.

Clearly from the above, the secondary pulse phenomena (as schematicallyillustrated in FIG. 2) begins when the bubble establishes its maximumvolume V_(m), since it is from this maximum value that the bubble beginsto collapse upon itself--or implode--resulting in the undesired,secondary acoustic peak.

The amplitude of this secondary pulse P₂, more precisely the product P₂×r, where r is the distance at which the pressure P₂ is measured, isdependent upon:

P_(H) --hydrostatic pressure

P_(m) =internal pressure of the bubble at V_(m) where V_(m) is themaximum volume of the bubble

If P_(m) =P_(H) the bubble will maintain its equilibrium state and nosecondary pressure pulse will be emitted, hence P₂ =0. However, thelarger the difference between P_(H) and P_(m), the greater the secondarypulse P₂.

2. Bubble Behavior

During the aforedescribed periodic oscillation of a bubble resultantfrom a gas charge, two forces act on the water. These forces include thegas pressure of the bubble P_(i), and the hydrostatic pressure P_(H).During the expansion phase of the bubble, the gas inside the bubbleundergoes an adiabatic expansion.

The law of conservation of energy applied to this expansion phase up tothe bubble's maximum volume V_(m), requires that the total work done bythe gas be equal to the work done by the hydrostatic pressure P_(H).(FIG. 13) Thus, any energy spent by an injected gas into the bubble mustbe compensated by an equal amount of extra work done by this hydrostaticpressure P_(H), which requires that the bubble's maximum volume beincreased proportionally. Restated, any injection of gas into the bubblebefore it reaches its maximum value V_(m), will result in a volumetricincrease or an increase in V_(m) to V'_(m). Therefore, any increase inthe internal pressure P_(i) of the bubble by injecting gas thereinbefore it reaches its original maximum value V_(m) will require acommensurately larger amount of energy which will have to be spent afterV_(m) and up to V'_(m). (FIG. 14)

3. Theory of Bubble Suppression

In the ideal case, total bubble suppression results in a strong firstprimary pulse with no secondary pulses, hence P₂ =0. To achieve P₂ =0,the bubble must reach its state of equilibrium characterized by twoconditions:

(a) When the bubble reaches its new maximum volume V'_(m) characterizedby dv/dt=0, or alternatively kinetic energy of the water E_(k) =0;

(b) then the internal pressure P_(i) of the bubble must be equal to theambient hydrostatic pressure P_(H) : P_(i) =P_(H) at V'_(m)

It has been found that if the above stated conditions are not satisfiedwhen the injection of the gas ends at a time t₂, it is not possible tolater satisfy these conditions. Restated, equilibrium must be achieved,if at all, at the end of the injection. If this equilibrium state is notreached, total bubble suppression of the secondary pulse is notpossible. Those two conditions (a) and (b) are independent of anyparticular way the injection is made.

If a first charge of highly pressurized gas having an energy E_(a) isexplosively released in a body of water, and if subsequently a secondcharge of a highly pressurized gas having an energy E_(b) is injectedinto the bubble (cavity) created by the first charge, and if the energylost by the injected gas during the injection is designated E_(i), thenwhen the injected bubble reaches it maximum volumetric value V'_(m) theinternal pressure of the bubble is given by: ##EQU2## where k=the ratioof specific heats of the gas utilized in the injection. For air k=1.40.

The variations in the dimensionless ration P'_(m) /P_(H) with respect toE_(i) is represented in FIG. 15. FIG. 15 illustrates that since totalbubble suppression (ideal) can only be achieved if P'_(m) =P_(H), totalsuppression can only be achieved when

    (k-1)E.sub.b /E.sub.a ≧1                            (4)

    or E.sub.b ≧E.sub.a /k-1

Since for air k=1.40, the injected energy E_(b) must be at least equalto 2.5 E_(a) ; hence for total bubble suppression:

    E.sub.b ≧2.5E.sub.a

For example, if the same pressure is used in both the first and secondgas charge (P_(a) =P_(b)), the ratio of the above values dictates thatV_(b) /V_(a) ≧2.5. But FIG. 15 shows further that if E_(b) is largerthan the minimum energy (represented by E_(a) /k-1), total suppressionis possible providing that the injection is made in such a way that theinjected gas fully utilizes or loses the energy E_(i), where ##EQU3## ora given proportion of the excess of the injected energy over the minimumenergy. As noted for air, k=1.40. Therefore k-1/k=0.286 and the energylost must be equal to 28.6% of the excess of energy.

It has also been found that the maximum volume V'_(m) of the bubble mustbe equal to ##EQU4## in order to achieve total bubble suppression.

Thus the two (2) conditions (a) and (b) above can be expressed by thefollowing relationship between the following parameters:

(c) total bubble suppression is possible only if the injected energy isat least equal to E_(a) /k-1: ##EQU5## and the total suppression isactually achieved if ##EQU6## and ##EQU7##

In other words, to establish total suppression, any injection of asecond charge of a highly pressurized gas into the bubble, where saidcharge has an energy E_(b), must be of such character that:

(d) the work done by the injected gas is equal to k-1/k times the excessof energy E_(b) -(E_(a) /k-1) over the minimum energy;

and

(e) the ratio of the new maximum volume to the original maximum volumeV'_(m) /V_(m) is equal to k-1/k times the ratio of the total energyspent (E_(a) +E_(b)) to the original energy that generates the bubbleE_(a).

FIG. 16 illustrates how these two basic conditions can be read on the(P,V) diagram. To establish total suppression, the injection must end atpoint (c) where P=P_(H) and V=V'_(m) given by equation (6) and the pathA→B→C followed by the injection must be such that the area under thecurve is given by equation (5), or equivalently equal to the area D, D',C, C'.

When the minimum energy E_(a) /k-1 is used, the relationships expressedin (5) and (6) also yield:

    E.sub.i =0 and V'.sub.m =V.sub.m

In this case, the injection must be instantaneous and must occur exactlyat V_(m) when the bubble reaches its original maximum volume V_(m). OnFIG. 16, the path followed by the injection is D→D': ideally theinjection starts at point D (V=V_(m)) and ends at point D' when theinternal pressure is P_(i) =P_(H).

This ideal case, (where the injected energy is exactly equal to theminimum energy E_(a) /k-1 and the injection is instantaneous and occursexactly at V_(m)), can be considered as a limiting case where theinjected energy is slightly larger than the minimum energy and theinjection occurs around V_(m). Such an injection can be represented onthe (P,V) diagram by a straight line passing through the point (V_(m),P_(H) /2) as indicated by the path A, B, C of FIG. 17. It can be seenthat such a path automatically satisfies the conservation of energy asindicated in FIG. 17: area (A, B, D)=area (B, C, D').

In this case, the injection starts at point A, where V₁ <V_(m) and endsat C, where V'_(m) >V_(m), and where both V₁ and V'_(m) are very closeto V_(m). Stated in terms of time, the injection starts just before T/2and ends just after T/2, while the volume of the bubble stays within afew percent of its maximum value.

It has also been found that in the above case, the energy E_(m) spentbefore and up to V_(m) is substantially equal to half the minimum energyor ##EQU8##

It has also been found that the energy spent before V_(m) and up toV_(m) increases only very slowly with the energy ratio E_(b) /E_(a).When this ratio is equal to its minimum value, (1/k-1), then E_(m) =1/2(E_(a) /k-1). E increases only by 14% when the energy ratio ismultiplied by 2, and therefore already represents an important loss ofefficiency in terms of energy for the overall process.

When the injected energy E_(b) is larger than the minimum value E_(a)/k-1, generally the injection starts before the bubble reaches itsinitial maximum volume V_(m), for instance, when its volume has a valueV₁ ≦V_(m), and ends when its volume reaches its new maximum value V'_(m)>V_(m) as indicated on the (P,V) diagram of FIG. 16 by the path A→B→C.Thus all possible injections leading to a total suppression must satisfythe above equations (5) and (6), which in turn impose constrains on thepossible ways the energy is released as a function of time.

FIG. 16 also illustrates that when only the work performed by theinjected gas and the maximum volume V'_(m) are considered, the pathA→B→C followed by any injection leading to total suppression isequivalent, and therefore can be replaced by the injection following thepath D→D'→C for which the injected energy is E_(b). The same is true forpath A→B→C; the work performed is E_(i) given by equation (5) and thenew maximum volume is V'_(m) given by equation (6). Restated, theequivalent injection starts at D. When the bubble reaches its maximumvolume V_(m), an amount of energy equal to the minimum energy E_(a) /k-1is injected instantaneously (Path D→D') to establish the hydrostaticpressure P_(H) within the bubble, and then the injection is made alongthe path D'→C at hydrostatic pressure, since this path follows areversible process it is consequently very slow.

In the equivalent injection, the minimum energy is instantaneouslyinjected into the bubble when the bubble is at its maximum volume V_(m),using a totally irreversible process (D→D'), during which no work isperformed by the injected gas. In this case, the excess of energy E_(b)-(E_(a) /k-1) is slowly injected into the bubble along a reversibleprocess (D'→C) for which the work performed is maximum and equal tok-1/k times the energy injected during this phase, or k-1/k times theexcess of energy E_(b) -(E_(a) /k-1). This equivalent injection dependsonly upon E_(b), if E_(a) is given, and therefore is the same for allthe possible injections leading to a total suppression while using thesame amount of injected energy E_(b).

The equivalent injection represents a limiting case of all injectionsleading to a total suppression and is neither economically efficient, asa total bubble suppression is already achieved at D', nor practical, asa dramatic change in the injection rate is required at D', but itillustrates that in fact any injection leading to total suppression canbe considered as a succession of two injections:

(1) one occurring around V_(m) and having an energy equal to about theminimum energy E_(a) /k-1; and,

(2) a second injection occuring after V_(m) up to V'_(m) and having anenergy equal to about the excess of energy E_(b) -(E_(a) /k-1.)

At any time during the injection, the energy E already injected into thebubble is split into two parts:

(1) The energy φ_(i) required to increase the internal pressure up toP_(i), and

(2) The work W_(i) performed by this internal pressure on the body ofwater:

    E=φ.sub.i +W.sub.i

It has been found that W_(i) is only a small fraction of E, or in otherwords, the main proportion of the injected energy spent is used toincrease the internal pressure of the bubble. More precisely, W_(i)varies from 0 (totally irreversible process) to the maximum value(k-1/k) E (reversible process). For air W_(i) is at the most equal to28.6% of the injected energy.

During the first phase of the injection which starts when the internalpressure is almost zero, the work performed is very small and almost allthe energy spent is used to raise the internal pressure. As any changein the bubble's behavior depends only upon the work performed on thebody of water by the internal pressure, it will be seen that during thefirst phase of an injection from volume V₁, when the injection starts,up to V_(m), the behavior of the bubble depends mainly upon the originalenergy released E_(a).

After V_(m), the original energy E_(a) has been transformed into thepotential energy P_(H) V_(m) of the body of water, and therefore afterV_(m), the behavior of the bubble depends only upon the injected energyE_(b) and, more precisely, upon the excess of energy E_(b) -(E_(a)/k-1).

Stated another way, the ideal injection must be of such character so as:

(1) to prevent any collapse of the bubble after V_(m) by establishingwithin the bubble an internal pressure close enough to P_(H) aroundV_(m) ; and,

(2) to use the excess of energy to increase the internal pressure toP_(H) and the volume to V'_(m).

It has been found both theoretically and experimentally that thecondition stated for a quasi instantaneous injection using the minimumenergy or slightly above the minimum energy holds for all values of theinjected energy.

In other words, in order to establish total suppression, any injectionmust be such that the amount of energy injected before the bubble passesthrough its maximum original volume V_(m) and up to V_(m) must besubstantially equal to half the minimum energy or 1/2 (E_(a) /k-1);##EQU9##

4. The Time the Injection Starts t₁

FIG. 18 represents the variations of the dimensionless ratio V'_(m)/V_(m) versus the energy ratio E_(b) /E_(a) according to equation (6).##EQU10##

As may be seen, the graph is a straight line passing through the pointsA₀ (0, k-1/k) and D'(1/k-1, 1), said line having a slope k-1/k.

When the injected energy increases above the minimum value E_(a) /k-1the point representing V'_(m) /V_(m) moves on the portion of the lineabove the starting point D' which corresponds to the point D' (V_(m),P_(H)) of the diagram illustrated in FIG. 16. But the diagramillustrated in FIG. 18 may also be used to plot the reduced volumeV/V_(m) versus E/E_(a) during the injection, where V is the volume ofthe bubble and E represents the energy injected at any instant duringthe injection. Such an injection leading to total suppression starts atpoint A where the volume is V₁ and the reduced volume V₁ /V_(m), passesthrough the volume V_(m), (reduced volume V_(m) /V_(m) =1) at point Band ends at point C for which the volume is maximum (dv=0) and has thereduced value V'_(m) /V_(m).

As seen in FIG. 18, the equivalent injection follows the path D→D'→Cwhere the path D→D' represents an instantaneous injection of the minimumenergy E_(a) /k-1. The path D'→C represent a reversible injection wherethe internal pressure remains equal to P_(H). Referring to this figure,it has been found that in accordance with the second law ofthermodynamics, the slope of the curve A→B→C cannot exceed k-1/k or theslope of the straight line. Accordingly, point A, when the injectionstarts, cannot be lower than the point A_(o) of ordinate k-1/k, or##EQU11## or translated into time by the well known relationship betweenV₁ /V_(m) and t₁ /T ##EQU12##

In other words no injection starting before t₁ =0.12 T can lead to anyquasi-total suppression.

Specifically, this is the case of the injections described in U.S. Pat.No. 3,653,460, as issued to Chelminski, where the described injectionsstart with the release of the first charge E_(a) at t=0, and therefore Ais practically 0. Experiments show that it is impossible with suchinjections to have a P₂ /P_(o) ratio less than 23%, which is generallyunacceptable.

Therefore, any ideal injection must start at a point A between A_(o) andD and follow a path A→B→C and end at C where the tangent to the path ishorizontal (dv=0). Furthermore the slope must never exceed k-1/k. Thoseconditions are narrowly limiting all possible paths, and for theconditions stated above, the equation: ##EQU13## defines the path whichintersects the line DD' at a point B for which: ##EQU14##

It has been found that the curve ABC must be very close to the straightline BC, and therefore point A where the injection starts can be foundsimply as the intersection of the ordinate's axis with the straight lineCB.

Expressed in terms of the energy ratio E_(b) /E_(a), the ordinate ofpoint A is found to be expressed by the equation: ##EQU15##

Translated into time by the known relationship between V₁ /V_(m) and t₁/T, when t₁ is the time when the injection starts: ##EQU16##

These relationships have been found to accurately coincide with allexperimental results up to the limit of the experiments for E_(b) /E_(a)=10.

When E_(b) increases to large values, t₁ /T tends toward a limit givenby: ##EQU17## For air: t₁ /T=0.23, which has been supported byexperimental results. In summation, 0.23≦t₁ /T≦0.50 where this P₂ /P_(o)ratio is equal or less than 5% (the quasi-ideal case.)

Since no assumption has been made on any special way the injection ismade, the above results are general and apply to all possible injectionsleading to a total bubble suppression. FIG. 48 represents the variationsof the more convenient dimensionless parameter (1/2-t₁ /T) as a functionof the energy ratio E_(b) /E_(a).

5. Duration of the Injection

Given the total energy released E_(b) and the time the injection startst₁, the duration Δt is the third parameter that characterizes aninjection.

As noted above, for the ideal injection the new maximum volume V'_(m)must be reached exactly at the end of the injection, emphasizing theimportance of the duration Δt. Also the ratio E_(b) /Δt represents theaverage rate at which the energy is injected as a function of time. Thisrate has been shown to be essential for ideal bubble suppression. Itwill be understood by those skilled in the art that the rate ofinjection E_(b) /Δt must be tuned to the rate of expansion of the bubbledv/dt that mainly depends upon the time t₁ at which the injectionstarts.

More precisely as shown in FIG. 20:

(a) If the rate at which the energy is released is too fast, and/or theinjection starts too late, the pressure will first increase above thehydrostatic value, for E_(b) >E_(a) /k-1 and finally at V'_(m) theinternal pressure will be below P_(H), resulting in to some kind ofsecondary pressure pulse. FIG. 20a.

(b) If the rate is too slow and/or the injection starts too early theinternal pressure will never reach the hydrostatic value P_(H) and asecondary pressure pulse will be emitted as a consequence of a certaindegree of collapse of the bubble. FIG. 20b.

(c) It is only when both the energy rate and the time the injectionstarts are correctly tuned that a total suppression can be achieved.FIG. 20c.

It has been found that the relationship between t₁, the time when theinjection starts, and the duration Δt of said injection, can be deducedfrom the conditions (a) and (b) and/or values of E_(i) and V'_(m) givenby equations (5) and (6), in terms of the energies E_(a) and E_(b).

It is well known from the prior art that when a bubble having a constantinternal pressure p, expands from a volume V₁ to a volume V'_(m), thetime Δt required to go from V₁ to V'_(m) is a known function of (p_(H)-p), V₁ and V'_(m).

For an ideal injection, V₁ and V'_(m) may be expressed in terms ofE_(a), E_(b) and V_(m) (see equation 6 or 7). An estimate of theduration of the injection Δt, can be made if it is assumed that betweenV₁ and V'_(m) the internal pressure of the bubble is constant and equalto its actual average value p=E_(i) /(V'_(m) -V₁) where E_(i) is givenby equation (5).

It has been further found that in order to establish total suppression,the time the injection starts t₁, and the injection's duration Δt, mustbe related by: ##EQU18## or according to equation (7) ##EQU19##

For air k=1.40 and the resulting coefficient is equal to 0.203.

Relationship (8) has been found to agree well with experimental resultsup to E_(b) /E_(a) =10.

In this way it has been discovered that, for an ideal bubble suppressionP₂ /P_(o) ≦5% the relative duration of the injection Δt/T must besmaller than or equal to K-1|3 times the energy ratio E_(b) /E_(a).##EQU20##

Restated, to achieve an ideal bubble suppression, the relative durationof the injection must at a maximum be equal to about K-1/3 times theenergy ratio. For air, ##EQU21## the relative duration must be smalleror at the most equal to about 14% of the energy ratio.

6. Preferred Embodiment

As no assumptions have been made on the way the energy is releasedversus time, it will be understood that the above conclusions regarding:

(a) the minimum energy injected E_(a) /K-1;

(b) the amount of energy to be injected before V_(m) or (1/2) E_(a)/k-1;

(c) the time the injection starts t₁ ; and

(d) the duration of the injection Δt are independent of the way theenergy is released.

Nevertheless, in the preferred embodiment the natural way of releasingthe injected energy is used. In this preferred embodiment, the secondcharge of a highly pressurized gas is stored under pressure P_(b) in achamber having a volume V_(b). The injected energy is proportional tothe product P_(b) V_(b).

At time t₁, the gas is allowed to flow into the bubble by the opening ofa fast acting valve, said valve remaining open until all the gas hasbeen transferred into the bubble. In this case the injection is definedby the three parameters:

(1) E_(b), the energy of the injected gas, which is proportional toP_(b) V_(b).

(2) θ, the time constant of the injection.

(3) t₁, the time at which the injection starts.

As known in the art, when a pressurized gas having pressure P_(b) andoccupying volume V_(b) is discharged from a given containment into avacuum, the pressure within the containment decreases according to theexponential law: ##EQU22##

And accordingly, the energy released follows the law ##EQU23##

In such a case, the time constant θ is a function of (1) the temperatureand the nature of the gas and (2) the ratio V_(b) /A_(b), where V_(b) isthe volume of the injector (earlier generically referred to as thecontainment), and A_(b) is the cross-sectional area or the orificethrough which the gas is flowing into said vacuum. For a given gas at agiven temperature, θ is proportional to V_(b) /A_(b) such that

    θ=c (V.sub.b /A.sub.b)                               (11)

where c is a constant expressed by ##EQU24## where N=coefficient of theorifice,

R=gas constant,

T=temperature.

It should be herein noted that θ does not depend on the pressure of thegas in the containment.

For practical applications, this time constant θ may be referred to as a"technical" parameter since several operating conditions may dictate therelative size of θ. For example, when the gas injection takes placeclose to T/2, gas must be rapidly evacuated from the injector. Thisrapid flow requirement dictates that the cross-sectional area A_(b) ofthe orifice in the injector must be relatively large. Hence and byreference to Equation (11), θ would be quite small. Specificallyreferring to this same example, however, when a very rapid injection ofgas is desired, gas flow through the orifice must take place in a matterof a few milliseconds--or less. Such instantaneous release and flowrequires technically sophisticated valves whose operation under suchconditions often leads to problems. Alternatively, a valve having asmaller area A_(b) is generally easier to operate.

In consequence of the above, to achieve ideal bubble suppression, theparameters E_(b), θ, and t₁ must satisfy the two conditions of (a) and(b) above, namely, P_(i) =P_(H) when dv=0. These absolute relationshipsenable the operator to arbitrarily select one given operating parameterand tailor the pulse signature utilizing the remaining two parameters.

For example, the time t₁ (defined as that time when the injectionstarts) is limited to the range 0.2 T-0.5 T (for ideal suppression),since the injection must at least begin before the bubble begins toimplode--this occurring at 0.5 T. Hence, t₁ may be arbitrarily chosen(within the stated range) by the operator. Alternatively, the gas energyE_(b) or the time constant of injection θ may also be randomly chosen bythe operator. As illustrated by both equation (7) and (8), as soon asone parameter E_(b), Δt, or t₁ has been chosen, the other parameters aregenerally determined.

The gas energy E_(b) represents a logical starting parameter since E_(b)ultimately represents the cost of the injection in terms of bothpressure and volume. For increasing pressures, larger and more powerfulcompressors will be necessary. Conversely, a larger volume V₁ requires alarger injector volume.

A. TUNING

(1) Ideal Suppression

It has been discovered both theoretically and experimentally, that anquasi ideal bubble suppression (i.e.--P₂ /P_(o) ≦5%) can be achieved byproperly "tuning" the parameters of injection, i.e. E_(b), θ, and t.These parameters are manipulated within the basic behavioral parametersof the bubble, i.e.--the energy of the bubble E_(a) =P_(H) V_(m) and theperiod of oscillation of said bubble or period T.

It has been further discovered that when such tuning is implemented,then, at the end of the injection at a time t₂, when all the energy ofthe injection has been transferred into the bubble, the bubble hasreached its (new) maximum volume V'_(m) and its internal pressure P_(i)=P_(H). It has been discovered that such tuning is possible for verybroad ranges of both the injected energy E_(b) and the time constant θ.In this, it has been discovered that E_(b) may have any value greaterthan a set minimum, and that θ may have any value as long as the ratioθ/E_(b) is smaller than a given maximum value. As noted, for totatlbubble suppression, t₁ must be greater than a minimum (0.2 T) andsmaller than 0.5 T.

For the preferred embodiment the time t₁ is given as in the general caseby relationship (7).

In this case and according to equation (9) the duration of the injectionfor which both the containment and the bubble are under the samepressure P_(H) is theoretically infinite. In practice Δt can be chosenequal to 2.2 θ, or 2.2 times the time-constant θ, which corresponds tothe time needed for the injected energy to grow form 10% of the totalenergy to 90% of the total energy E_(b).

Setting Δt=2.2 θ in formula (8) for air, will lead to: ##EQU25##

The third term after the coefficient decreases only slowly when theenergy ratio increases from its minimum valve 2.5, and this term can beconsidered as a correcting term. When the energy ratio increases fromits minimum value 2.5 at the beginning, the dominant term is the secondterm and therefore θ/T increases rapidly, more rapidly than E_(b)/E_(a).

When the ratio E_(b) /E_(a) reaches a value of about 5, the valuerepresented by the second term levels off and θ/T is almost proportionalto E_(b) /E_(a) : ##EQU26## which represents the tangent from theorigin.

If the increase of injected energy E_(b) is due to the increase of thevolume V_(b), the pressure P_(b) remaining constant, then (11) can bereformulated: ##EQU27##

It can be seen that when the energy ratio E_(b) /E_(a) is between 2.5and about 5, θ must increase faster than E_(b), and thus thecross-section area A_(b) must be decreased.

When the energy ration E_(b) /E_(a) is equal or larger than about 5,(11') illustrates that A_(b) must remain constant, and the increase of θneeded for tuning will be solely a consequence of the increase in thevolume V_(b) needed to increase the energy E_(b). This rather surprisingbehavior has been established experimentally.

It has been discovered both theoretically and experimentally that:

(a) When the injected energy E_(b) is between the minimum energy E_(a)/k-1 and about twice this minimum 2E_(a) /k-1 or for air, for example,when E_(b) is between 2.5 E_(a) and 5E_(a), then the area A_(b) must bereduced when the energy E_(b), or the volume V_(b) increases, to staytuned to the energy.

(b) When the injected energy E_(b) is larger than about twice theminimum energy E_(a) /k-1, then the value of the time constant θ forwhich a total bubble suppression can be achieved is about proportionalto V_(b). Therefore, the area A_(b) must remain constant when theincrease in energy is obtained by an increase of the volume V_(b) at aconstant pressure P_(b).

(c) The relative time constant θ/T is smaller than or equal to about0.06 times the energy ratio E_(b) /E_(a) for air, or generally (k-1/6.6)times E_(b) /E_(a).

All tests achieving results P₂ /P_(o) ≦5% closely follow the linesrepresenting the relationships (7) and (12) as indicated in FIGS. 19 and20.

By way of example only, in a series of experiments the cross-sectionarea A_(b) was reduced from 6.14 in² (40 cm²) to 0.85 in² (5 cm²) whenthe energy ratio was increased from about 2.5 to 5. The cross-sectionalarea was kept constant at 0.85 in² (5 cm²), while the energy ratio wasincreased from 5 to 10, by increasing the volume V_(b) while thepressure P_(b) remained constant at about 2000 psi (140 bars) accordingto equations (8) or (12) as indicated in FIG. 20.

Referring to FIGS. 22 and 23, it is shown how the three parameters, (1)the relative time constant θ/T, and (2) the ratio of energy E_(b) /E_(a); and the time t₁, can be modified or "tuned" to achieve an acceptableP₂ /P_(o) ratio.

EXAMPLE 1

In a first experiment a E_(b) /E_(a) ratio of 2.33 was achieved, where

    ______________________________________                                        P.sub.a = 2000 psi  P.sub.b = 2000 psi                                        V.sub.a = 45 in.sup.3                                                                             V.sub.b = 105 in.sup.3                                    ______________________________________                                    

This ratio is smaller than the minimum energy ratio of 2.5 earlierrecited. For this experiment, the relative time constant was θ/T=0.073yielding a point A as seen in FIG. 22. As seen in this Figure, point Ais above curve C for ideal tuning, where C represents the equation (12).

In this example, before tuning, P₂ /P_(o) was equal to 9%. By increasingthe pressure in the injector's chamber from P_(b) =2000 psi to P_(b)=2500 psi, the energy ratio was brought to E_(b) /E_(a) =2.92. Therelative time constant θ/T remained constant at 0.073. The tunedimprovement is seen at point A', and the ratio P₂ /P_(o) has beenreduced to 4%.

EXAMPLE 2

In a second experiment where,

    ______________________________________                                        P.sub.a = 2000 psi  P.sub.b = 2500 psi                                        V.sub.a = 45 in.sup.3                                                                             V.sub.2 = 210 in.sup.3                                    ______________________________________                                    

A ratio E_(b) /E_(a) was established at 5.83 where the ratio θ/T=0.28.This value is represented at point B in FIG. 22, well below ideal curveC. Using the above parameters, a P₂ /P_(o) ratio was established at 17%.By decreasing the pressure of the injected gas P_(b) from 2500 psi to2000 psi, the E_(b) /E_(a) ratio is decreased from 5.83 to 4.66, againwithout changing the ratio θ/T. After correction, the P₂ /P_(o) ratiowas decreased from 17% to 5%. This "tuned" value is represented at pointB' in FIG. 22.

EXAMPLE 3

In a third experiment, where

    ______________________________________                                        P.sub.a = 2500 psi  P.sub.b = 2500 psi                                        V.sub.a = 45 in.sup.3                                                                             V.sub.b = 210 in.sup.3                                    ______________________________________                                    

A E_(b) /E_(a) ratio of 4.66 was established yielding a relative timeconstant of θ/T=0.25, due to the increase of the period T with thepressure in the generator (P_(a) =2500 psi). The P₂ /P_(o) ratio wasestablished at 12%. The representative point for this valve isrepresented at point B" in FIG. 23. By decreasing both pressure P_(a)and P_(b) down to P_(a) =2000 psi, B" was effectively "tuned" to wherethe ratio E_(b) /E_(a) was unchanged and equal to 4.66. In this casealso, the period T was reduced from T=98 msec down to T=90 msec so thatthe ratio θ/T was increased from θ/T=0.25 to θ/T=0.28. As shown in FIG.23, this "tuning" results in a new P₂ /P_(o) ratio of 5% as shown atpoint B'.

As illustrated in the above examples, there exists a variety of methodsto improve or "tune" a given pressure pulse proximate to the desiredθ/T, t₁ /T and E_(b) /E_(a) values, so as to result in a commerciallyfeasible P₂ /P_(o) value.

To summarize:

When the ratio θ/T is properly tuned to the energy ratio E_(b) /E_(a),then the time when the injection starts or more conveniently the ratiot₁ /T needs to be optimized or tuned to the same energy ratio E_(b)/E_(a).

By way of example, FIG. 24 illustrates how the amplitude ratio P₂ /P_(o)changes with the value of t₁ /T. In this example:

    ______________________________________                                        P.sub.a = 2000 psi  P.sub.b = 2000 psi                                        V.sub.a = 45 in.sup.3                                                                             V.sub.b = 280 in.sup.3                                    E.sub.b /E.sub.a = 6.22                                                                           θ/T = 0.38                                          ______________________________________                                    

The optimum tuning is reached when t₁ /T=0.24 and then P₂ /P_(o) =3.5%.

It should be noted in FIG. 24 that a relatively wide range of variationsfor t₁ /T: from 0.19 to 0.27 are allowable and the ratio P₂ /P_(o) stillremain below or equal to 5%.

Time Constant

θ/T can be increased (or decreased) by changing the area A_(b) by whichthe injected gas is flowing from the injection chamber. In such a case,T, E_(a) E_(b) will remain constant. This can be accomplished by varyingthe cross-sectional area of the orifice at the outlet of the injectionchamber itself.

Energy of the injector

E_(b) can be increased (or decreased) by increasing (or decreasing) thevolume V_(b) while the pressure P_(b) remains constant. In such a case,θ and therefore θ/T, will increase proportionately.

E_(b) can also be increased (or decreased) by increasing (or decreasing)the pressure P_(b) while the volume V_(b) remains constant. In such acase, θ will remain constant.

Energy of the Generator

E_(a) can be increased (or decreased) to change the ratio E_(b) /E_(a).When E_(a) is changed, however, T will also vary according to therelationship T=m (E_(a))^(1/3). In such a fashion, the ratio θ/T willsimilarly vary like E_(b) /E_(a), but at a different rate.

Period

The period T can also be used for the tuning of the ratio θ/T or t₁ /Tto E_(b) /E_(a).

Time for Injection, t₁

As earlier noted, to achieve complete suppression gas injection mustbegin before 0.5 T (or at 0.5 T assuming instantaneous injection) assoon as E_(b) /E_(a) =2.5. When the energy of the injected gas is closeto its minimum value, the volume of the bubble must be at its maximumvalue or very close to it. Near the maximum volume, the volume of thebubble varies only very slowly with time. For example, during the periodof time from 0.4 T to 0.6 T, the volume of the bubble remains within 5%of its maximum value. If the injection occurs rapidly enough, (ideallyinstantaneous), the time at which this injection is made can vary in arelatively wide range, for instance, bewteen 0.4 T and 0.6 T, due to thestability of the volume when it is close to the maximum. Consequentlythe time t₁, or more conveniently the ratio t₁ /T, will vary rapidlyaround the minimum energy ratio E_(b) /E_(a) =2.5 (for air).

The applicant has discovered both theoretically and experimentally thatdue to the interference between the energy of the injected gas E_(b),the time constant θ, the value V₁ reached by the bubble when theinjection starts at time t₁, and the velocity V₁ (dv₁ /dt) at time t₁,to achieve a commercially viable bubble suppression t₁ decreases to aminimum value at about 0.23 T. Therefore, for complete bubblesuppression (P₂ /P_(o) ≦5%), the time t₁ when the injector starts shouldpreferably begin within the range 0.2 T and 0.5 T.

(2) Acceptable Bubble Suppression

It has been found that the above conditions for quasi-ideal suppressionmay be widely extended should the ratio P₂ /P_(o) be allowed to increaseto an acceptable level higher than 5%.

By way of example it can be seen on the experimental curve of FIG. 24that for an energy ratio E_(b) /E_(a) =6.22 and a correctly tunedrelative duration Δt/T (or relative time constant θ/T, the time theinjection starts t₁ /T when ideally tuned at t₁ /T=0.24 leads to a P₂/P_(o) ratio of P₂ /P_(o) =3.5%.

But when the ratio P₂ /P_(o) is allowed to reach 10% or less, then theratio t₁ /T can be chosen within the limits 0.12 T-0.35 T as illustratedon FIG. 24.

Should the ratio P₂ /P_(o) be allowed to stay below or equal to 15%, thesame ratio t₁ /T would have a range 0.05 T-0.35 T as illustrated on FIG.24.

In order words the relationship (7) and the representation of FIG. 19,are ideal or quasi-ideal conditions leading to a total bubblesuppression or P₂ /P_(o) equal or near zero P₂ /P_(o) ≃0.

When the ratio P₂ /P_(o) is allowed to increase--for instance up to 15%,which represents an average value of what an air gun array yields instandard condtions--then the curve of FIG. 19 should be replaced by anallowable zone extending on both sides of the curve, or a strip, and thewidth of the strip will depend upon the maximum range allowable for theratio P₂ /P_(o).

By reference to FIG. 24, the general shape of the curve representing thevariations of the ratio P₂ /P_(o) versus t₁ /T, holds for all energyratio E_(b) /E_(a) and all values of the ratio Δt/T or θ/T. But theordinate of the vertex, or the minimum value of the ratio P₂ /P_(o),will depend upon the tuning of θ/T to the energy ratio E_(b) /E_(a).

By way of example, reference is made to the following experiment:

    ______________________________________                                        P.sub.a = 2,000 psi P.sub.b = 2,000 psi                                       V.sub.a = 45 in.sup.3                                                                             V.sub.b = 150 in.sup.3                                    E.sub.b /E.sub.a = 3.33                                                                           θ/T = 0.21                                          ______________________________________                                    

for which A_(b) =0.85 in², the optimum value for t₁ /T was found to bet₁ /T=0.30 and P₂ /P_(o) has a minimum value at P₂ /P_(o) =8%.

For E_(b) /E_(a) =3.33 the curve of FIG. 21 gives an ideal value for θ/Tequal to 0.15, lower than the actual measured value of 0.21, indicatingthat the actual injection's rate was not large enough.

By increasing the cross-section area A_(b) from A_(b) =0.85 in² to A_(b)=1.10 in² a minimum ratio P₂ /P_(o) =4% was achieved for a time ratio t₁/T=0.30, where the ratio θ/T was established at 0.17 closer to the idealvalue of 0.15.

As shown in the above example the relationship (8) or (12) and therepresentation of FIG. 21, are the ideal conditions leading to P₂ /P_(o)≃0. Whenever the ratio P₂ /P_(o) is allowed to increase the curve ofFIG. 21 may be replaced by an allowable zone or strip extending on bothsides of the curve, the width of which will depend upon the maximumrange allowed for the ratio P₂ /P_(o).

As it will appear to those skilled in the art, the only comparablesingle source, the air gun, yield an average bubble to primary ratio P₂/P_(o) of about 70%.

As said above an average value for the new-used air gun arrays can beestablished at about 15% in standard conditions, but the actual value ofP₂ /P_(o) from shot to shot is rather erratic as a result of dependingupon the tuning of a multitude if different sources.

With a source according to the invention, in many instances a ratio of15% or even 20% is acceptable, because the oscillating tail represents asmooth, monochromatic, stable oscillation as compared to the violentbehaviour of an air gun's oscillation.

This mono frequency signal is useful for very deep penetration where allother higher frequency components have been absorbed by the earth, andcan be easily processed as a single frequency signal.

It will be apparent to those skilled in the art that according to theabove disclosure the terms tuning or adjusting should be understood inlight of a previous choice of a maximum value allowable for the ratio P₂/P_(o).

In other words the tuning between the three parameters E_(b) /E_(a), θ/Tand t₁ /T must be considered in accordance with the maximum value of theration P₂ /P_(o) to be achieved.

As recited earlier the conditions for ideal bubble suppression are:

(a) when the bubble reaches its new maximum volume, characterized bydv=o then,

(b) the internal pressure must be equal to the ambient hydrostaticpressure P_(H),

As noted, the above conditions must be reached at the end of theinjection.

For a non-ideal injection it could happen that when the volume stopsincreasing (dv=o) and the internal pressure is about the hydrostaticpressure a part of the energy, normally small compared to the totalenergy of the injection, remains to be injected into the bubble. Thisinjection's tail will increase the size of the bubble while the internalpressure remains about the hydrostatic value P_(H), this phase of theinjection following an almost reversible process represented on FIG. 20Cby the path C→E, where the volume is increased from V'_(m) to V₄₁ _(m).

Because in this case the volume V'_(m) is no longer a maximum but ratheris stationary (with dv=o), the term "stationary" will be herein used asa more general term than "maximum". When the volume is stretched fromV'_(m) to V"_(m), normally the rate at which the volume increases issmall and therefore no significant pressure pulse is emitted during thisphase.

In this way, an excess of energy can be dissipated provided that theideal conditions (a) and (b) have already been established. FIG. 25illustrates this behavior, where at time 0.08 the internal pressurereaches the hydrostatic value P_(H) and remain there for a fewmilliseconds before the bubble resumes its expansion, eventuallyemitting a secondary pressure pulse that does not exceed 2% of theprimary pulse.

So as shown in this example, for the condition of a given injection t₁/T and θ/T, the energy ratio can vary within a range that depends uponthe acceptable P₂ /P_(o) ratio. Typically a 20% variation the energyratio can be allowed and can be dissipated by the stretching of thebubble, resulting in a corresponding increase in volume.

Basically when the three basic parameters E_(b) /E_(a), t₁ /T and θ/Tand not ideally tuned, the first consequence will be that when thevolume reaches its new maximum value V'_(m), the internal pressureP'_(m) is less than the hydrostatic pressure P_(H), and therefore thebubble will start to collapse and therefore emit a secondary pressurepulse.

The magnitude of this secondary pressure pulse will depend on themagnitude of the difference p_(H) -P'_(m). For given values of E_(b)/E_(a), t₁ /T and Δt/T, the internal pressure will reach a value ofP'_(m) when the volume reaches its maximum value V'_(m). Therefore, ifP'_(m) ≠P_(H), a secondary pressure pulse will be emitted, with aamplitude ratio P₂ /P_(o) being a function of the established basicparameters E_(b) /E_(a), t₁ /T and Δt/T.

It has been found theoretically and experimentally that the ration P₂/P_(o) depends upon the parameters of the injection E_(b) /E_(a), t₁ /Tand Δt/T by a relationship in the form:

    P.sub.2 /P.sub.o =(P.sub.H /P.sub.a).sup.2/3 ƒ (E.sub.b /E.sub.a, t.sub.1 /T, Δt/T)

It has been surprisingly discovered that the same type of relationshipholds when no injection is performed (as no injection can be consideredas a special injection for which E_(b) =o), and therefore holds true forthe use of a single air gun.

All published results on the measurement of air gun signatures when thefiring pressure P_(a), the hydrostatic pressure P_(H), or the depth arechanged, display the effect of this factor (P_(H) /P_(a))^(2/3).

For instance, the following table illustrates the value of the product(P₂ /P_(o))×(P_(a) /P_(H))^(2/3) for a single air gun having a volumeV_(a) =45 in³ and a firing pressure P_(b) =2,000 psi, fired at differentdepths (P_(H)):

    ______________________________________                                        (m)    P.sub.2 /P.sub.o                                                                          P.sub.a /P.sub.H                                                                      (P.sub.2 /P.sub.o) × (P.sub.a /P.sub.H)2/                               3                                                  ______________________________________                                        1.5    0.59        120     14.2                                               3      0.64        107     14.0                                               6      0.72        87      14.4                                               9      0.80        73      13.6                                               12     0.86        63      13.8                                               15     0.87        56      13.0                                               ______________________________________                                    

This table shows that the product remains constant at 13.8±5%.

In the above example the effect of the ratio (P_(a) /P_(H)) is toactually decrease the ratio P₂ /P_(o) when the firing depth is reduced.

In the case of a single air gun the magnitude of P₂ /P_(o) is large andtherefore the effect of (P_(a) /P_(H))^(2/3) is of no consequence. Butwhen the bubble ratio P₂ /P_(o) has been already reduced according tothe above disclosure, then a further reduction can be achieved byreducing the ratio P_(H) /P_(a), more specifically by increasing P_(a),or decreasing P_(H) --or the firing depth--or both. For example when thepressure in the generator Pa is increased from 2000 psi to 5000 psi, atthe same depth the ratio P₂ /P_(o) can be reduced by a factor(5000/2000)^(2/3) =1.8. Also by firing the same source at 5 ft. (1.5M)instead of 50 ft. (15M), a reduction by a factor of (2.5/1.15)^(2/3)=1.6 can be expected for the ratio P₂ /P_(o).

As shown in the above disclosure, the three injection's parameters E_(b)/E_(a), t₁ /T and Δt/T (or θ/T) can be tuned to achieve almost any valueof bubble suppression as required for any desired use.

In practical applications it could happen that detuning effects of somehidden parameters seems to alter the conculsions of the presentinvention.

For example and as earlier noted, the energy E_(b) represents the energyactually injected within the bubble. In some instance, the energyactually injected within the bubble could be substantially differentfrom the energy stored in the suppressor. This occurs, for example, iftoo large a portion of the energy stored in the suppressor is spent ordissipated in friction or turbulence before actually being injected intothe bubble. A similar event might be true if a substantial part of theenergy stored is used to activate the valve.

Additionally, the time t₁ when the injection starts represents the timewhen a substantial amount of gas starts to flow into the bubble itself.It could be the case, however, that this instant is somewhat differentfrom the time the suppressor starts to depressurize. This might occur iftoo large a dead volume is located between the suppressor and the bubbleitself, or if the flow is delayed by the friction caused by the designof the injector.

Further the duration of the injection could be altered by the behaviorof the opening valve or by a restruction occurring in the fluid flow,etc.

Thus although the value of some of the parameters may vary with thespecific technology used to embody the present invention, it will beunderstood by those skilled in the art that they will remain in thescope of the invention as disclosed above.

Acoustic Generator

The acoustic marine source of the present invention may be illustratedby reference to FIGS. 5-12A. In the claimed invention one can employ,for example, a conventional explosive signal generator 10 (FIG. 5) forgenerating at a predetermined and sufficient depth within a body ofwater a signal explosion, which produces at time t=o, a bubble 1 and adesired primary pulse having an amplitude P_(o) (FIGS. 1, 2). In theabsence of any suppression as previously described, bubble 1 wouldundergo a series of implosions and explosions (FIG. 1) at an oscillatingperiod T, which would result in the undesirable secondary peaks (P2, P4. . . ) of decreasing amplitude (FIG. 2).

To abort the first implosion, a suppressor generator 10' is utilizedwhich injects a charge of highly pressurized gas inside bubble 1. Thisinjection is generated within a time internal satisfying theaforereferenced conditions.

Explosive signal generator 10 can be a commercially available air gun,such as the one manufactured under the trademark PAR, which is fullydescribed in U.S. Pat. No. 3,379,273. Such an air gun has a signalchamber 14 whose volume Va is charged up with pressurized air G1.Chamber 14 communicates directly with an explosive shuttle valve 12 thatcan be actuated to explosively discharge the pressurized air G1 fromsignal chamber 14 into the surrounding body of water through dischargeports 42.

Valve 12 includes a main piston 70 engageable with a seal 31 forretaining a charge of pressurized gas G1 within signal chamber 14, and acontrol piston 72 engageable with a seal 71 for controlling themovements of piston 70. Pistons 70 and 72 are held together, in spacedparallel relation, by a hollow shaft 70' having an axial bore 33therethrough.

A compressor on the deck of the seismic vessel (not shown) supplies airpressure to input line 22 at 2000 psig, which is fed to a control orreturn chamber 32 from which it passes through a metering orifice 44 andaxial bore 33 into signal chamber 14.

The actuation of valve 12 is controlled by a controller such as asolenoid-operated valve 20, which is energized periodically by a shortelectric pulse produced by a conventional electronic actuating network(not shown) located on the deck of the seismic vessel. The firing ofexplosive generator 10 is periodically repeated as dictated by thefiring cycle of the seismic survey.

When solenoid valve 20 is fired, pressured gas flows from a line 22through a trigger passage 39 leading to the opposite surface of controlpiston 72 from that facing control chamber 32. Thus, the holding forceof the air pressure in control chamber 32 becomes instantaneouslyoffset, allowing the pressurized gas G1 in the signal chamber 14 tosuddenly accelerate main piston 70 away from its seal 31, therebysuddenly opening the discharge ports 42 and allowing them to communicatedirectly with signal chamber 14.

Then, the pressurized gas G1 from signal chamber 14 is explosivelyreleased through discharge ports 42 into the surrounding water whereinit generates a long acoustic seismic wavelet or pressure signature (FIG.2) having the desired acoustic primary Pulse P_(o), which is followed bythe undesired positive (P2, P4 . . . ) pressure pulses.

After the discharge of gas G1 from signal chamber 14, the pressure incontrol chamber 32 returns shuttle 12 to its closed position, andgenerator 10 is ready for a new cycle.

A deflector 15 removably couples then together. A removable plate 67having an orifice--disposed at its center of area A_(b), is secured tothe deflector 15. The area A_(b) can be easily changed (by interchangingthe plates) to "tune" the time constant θ of the discharge of thesuppressor 10'b, with the suppressor volume and pressure. Other means tovary this area A_(b) are also envisioned, such as a ball valve, abutterfly valve, etc. The cylindrical housing 73 of deflector 15 definessignal chamber 14 as well as four outlet ports 66 (FIGS. 5, 9), whichpreferably make a 60° angle with the vertical or longitudinal axis andpreferably are angularly aligned with discharge ports 42. Signal chamber14 of generator 10 is charged up from inlet line 22.

The suppressor generator 10'b is more fully described in French patent2,158,730. Housing 74 of generator 10'b defines a suppression chamber14'b which is filled from inlet 22'. Valve 12'b slides on a piston 52and is pneumatically operated through inlet lines 57 and 58 which aredifferent pressures. Inlet 57 supplies the triggering chamber 54 with1000 psi, and inlet 58 supplies the return chamber 56 with 70 psi.

At the start, a pulse signal is sent on line 11 to solenoid 20 whichallows valve 12 to open explosively. A delayed electric signal is thensent to solenoid valve 20' on line 11' inside conduit 58. The delay is 5ms.

When activated, valve 12'b opens to allow air G2 from suppressionchamber 14'b to become rapidly released into bubble 1 through ports 66of deflector 15.

Chamber 14 is repressurized after 60 ms, and chamber 14'b isrepressurized after 1 second.

The minimum ratio PbVb/PaVa is equal to about 1.8 where Vb is the volumeof chamber 14'b, and Pb is the pressure of the gas filling it. Pa is thepressure of the gas filling said chamber (14) and Va is the volume ofchamber 14.

I. DETAILED DESCRIPTION OF ONE OPERATING CYCLE PHASE 1. Generator 10Ready to Fire

Generator 10 (FIGS. 5, 6) and generator 10'b are armed.

Solenoid valves 20 and 20' are closed.

Shuttle valves 12 and 12'b seal off respectively signal chamber 14 andsuppression chamber 14'b.

Source B is pressurized:

P14=P33=P32=2000 psi.

P14'6=3000 psi.

P39=P55=Ph (hydrostatic pressure)=17 psi.

P56=105 psi.

P54=1500 psi.

Volumes:

Va=14+33=45 in³.

Vb=14'b=150 in³.

Area A_(b) =0.85 in².

Eb/Ea=5.

PHASE 2. Generator 10 Exploded and P_(o) Generated

At time t=0 (FIGS. 7, 7A).

Firing is initiated by energizing solenoid 20 of generator 10 with ashort electric pulse on line 11.

Shuttle 12 moves up explosively to allow the compressed gas G1 fromsignal chamber 14 to discharge through ports 42 into the surroundingwater.

This explosive air release generates the primary acoustic pulse P_(o) onthe pressure signature.

Bubble 1 is expanding (FIG. 3).

PHASE 3. Bubble 1 Is Still Expending and Encompasses Ports 66

At t=20 msec (FIGS. 8, 8A).

Bubble 1 encompasses ports 66.

Pressure inside bubble 1 is much less than hydrostatic pressure;actually the bubble, at this instant, can be considered as a vacuumcavity.

A negative pulse P1 appears on the pressure signature.

Generator 10'b is still in stand-by.

PHASE 4. Generator 10'b is Opened-Gas G2 Injected Into Bubble 1

At t₁ =27 msec (FIGS. 9. 10, 10A).

After a delay of 27 msec, depending upon the volume V_(a) pressure Pa ofthis gas in chamber 14, water depth, and/or volume and air pressure inchamber 14'b, solenoid 20' is also energized via line 11'.

The gas G2 in triggering chamber 54 is sent to chamber 55 throughsolenoid valve 20'.

The force on shuttle 12'b reverses, causing it to explosively movedownwardly.

The gas G2 from suppression chamber 14'b is released into bubble 1through orifice in plate 67 deflector ports 66 which are encompassedthereby.

The acoustic pressure signal shows an upward slope on the pressuresignature.

PHASE 5. Hydrostatic Pressure Established Inside Bubble

At t₂ =63 ms (FIGS. 11, 11A).

Eb/Ea, A_(b), and t₂, were selected so that gas in chamber 14'b canestablish hydrostatic pressure inside bubble 1 at this time.

The amplitude of the acoustic signal is zero.

Shuttle 12 of generator 10 moves downwardly and seals against ring 31,because of the 2000 psi pressure in return chamber 32.

PHASE 6. Bubble 1 Is Stabilized At Its Equilibrium Position

The implosion of bubble 1 has been aborted.

Bubble 1 is stabilized and its boundary undergoes very low amplitudeoscillations (FIGS. 12, 12A) which generate a negligible low-amplitudeacoustic signal. P₂ /P_(o) =2%.

Shuttle valve 12'b recocks because of the pressure in return chamber 56.Pressure in triggering chamber 54 is vented outside through a smallorifice (not shown).

Source B is repressurized and made ready for another cycle.

What is claimed is:
 1. A method for generating within a body of water anacoustic signal, comprising:explosively releasing within said body ofwater at a time t=o a first charge of a highly pressurized gas having anenergy E_(a) so as to produce in said water a primary pressure pulse anda very low pressure region tending to oscillate at a period T and togenerate a secondary pressure pulse; injecting within said region asecond charge of a pressurized gas having an energy E_(b) such thatE_(b) is at least equal to 1/2 (E_(a) /k-1) where k equals the ratio ofthe specific heats of the injected gas; and, tuning the duration of saidinjection and the time when the injection starts so as to substantiallyestablish hydrostatic pressure within said region at about the instantthe volume of said region reaches a stationary value.
 2. The method ofclaim 1 where said injection of said second charge starts between 0.05 Tand 0.5 T.
 3. The method of claim 1 where said duration of saidinjection has a maximum value substantially equal to about (k-1/3 (E_(b)/E_(a)) times the period T.
 4. A method for generating within a body ofwater an impulsive acoustic signal, comprisingexplosively releasingwithin said body of water at a time t=0, a first charge of a highlypressurized gas, having an energy E_(a) so as to produce in said water apowerful primary pulse and a very low pressure region tending tooscillate at a period T and to generate secondary pulses; injectingwithin said region a second charge of a pressurized gas, said secondcharge having an energy E_(b) sufficient to establish hydrostaticpressure within said region, where said injection of said second chargebegins substantially between 0.05 T-0.5 T; and adjusting the ratio ofthe duration of said injection to the period T such that the duration ofsaid injection has a maximum value substantially equal to the product ofthe period T and (k-1)/3 (E_(b) /E_(a)), where k is the ratio of thespecific heats of the injected gas.
 5. The method of claim 4 where saidfirst charge has a volume V_(a) and a pressure P_(a), said second chargehas a volume V_(b) and a pressure P_(b), such that the ratio P_(b) V_(b)/P_(a) V_(a) has a value greater than or equal about 2.0.
 6. The methodof claim 4, wherein said impulsive acoustic signal has a power spectrumflat substantially within 10dB over a frequency range of four octaves.7. An acoustic source for generating within a body of water selectedimpulsive acoustic signals, comprising:means for explosively releasingwithin said body of water at a time t=0 a first charge of a pressurizedgas having an energy Ea so as to produce in said water a selectedprimary pressure pulse and a low pressure region, said region tending tooscillate at a period T and to produce undesired secondary pressurepulses; means for injecting within said region a second charge of apressurized gas, said second charge having an energy Eb sufficient tosubstantially establish hydrostatic pressure within the region; meansfor adjusting the duration of said injection such that said duration hasa maximum value substantially equal to the product of the value (k-1)/3(E_(b) /E_(a)) and the period T, where k equals the ratio of thespecific heats of the gas of the second charge; and means for adjustingthe time the injection begins between a time 0.05 T and 0.5 T.
 8. Theacoustic source of claim 7 where prior to said injection, the volume ofsaid region increases toward a maximum value V_(m), the source furthercomprising means for tuning the time when said injection starts so thatthe quantity of energy that has been injected within said region whenthe volume of said region is equal to V_(m) will be substantially equalto 1/2 E_(a) /k-1, where k equals the ratio of specific heats of the gasof the second charge.
 9. The acoustic source of claim 7 furthercomprising means for periodically repeating said explosive release ofsaid pressurized gas and said injection of said pressurized gas.
 10. Theacoustic source of claim 7 wherein said first charge has a volume V_(a)and pressure P_(a), said second charge has a volume V_(b) and a pressureP_(b) and the ratio P_(b) V_(b) /P_(a) V_(a) has a value greater than orequal about 2.5.
 11. A repetitive marine acoustic source for generatingwithin a body water impulsive acoustic signals, comprising:a signalchamber having discharge port means into the water, said signal chambercontaining a first charge of a pressurized gas having an energy Ea; afirst fast-acting valve means normally maintaining said signal chamberclosed; an injection chamber having outlet means, said injection chambercapable of containing a second charge of a pressurized gas having anenergy Eb; a second fast-acting valve means normally maintaining saidinjection chamber closed; control means for opening said firstfast-acting valve means at a time t=0, so as to explosively dischargesaid first charge through said signal chamber's port means into saidwater, thereby generating within said water a primary pressure pulse anda low pressure region, said low pressure region tending to oscillate ata period T and to produce in said water secondary pressure pulses; saidcontrol means opening said second fast-acting valve means, therebyreleasing said second charge through said injection chamber's outletmeans into said region; means for adjusting the duration of saidinjection such that said duration has a maximum value substantiallyequal to the product of the value (k-1)/3 E_(b) /E_(a) and the period T,where k equals ratio of the specific heats of the gas of the secondcharge, E_(a) equals the energy of the first charge, and E_(b) equalsthe energy of the second charge; and means for controlling the time theinjection begins between substantially 0.05 T and 0.5 T.
 12. The marineacoustic source of claim 11 further including means for periodicallyrepeating said explosive discharge and said injection.
 13. The marinesource according to claim 11, wherein said outlet means communicatesdirectly with said water.
 14. The marine source of claim 11, whereinsaid outlet means are encompassed by said region.
 15. The acousticsource according to claim 11, wherein said control means includepneumatic delay means.
 16. The marine source according to claim 11,wherein said control means include electrical delay means.
 17. A methodfor generating within a body of water an impulsive acoustic signalhaving a primary pressure pulse with an amplitude P_(o), and tending tohave secondary pressure pulses with amplitudes P₂,comprising:explosively releasing within said body of water a firstcharge of a highly pressurized gas at a time t=0, where said gas has avolume V_(a) and a pressure P_(a) so as to produce a powerful primarypulse and a cavity, said cavity tending to oscillate at a period T;injecting a second charge of highly pressurized gas having a volumeV_(b) and a pressure P_(b) within said region at a time before thecavity begins to implode such that the product P_(b) V_(b) has a minimumvalue at least equal to 1/2 (P_(a) V_(a) /k-1); and adjusting thepressure P_(b) of the second charge so that the ratio of the secondarypulse P₂ to the primary pulse P_(o) achieves a range of 0-15%.
 18. Amethod for generating within a body of water an impulsive acousticsignal having a primary pressure pulse with an amplitude P_(o) andtending to form secondary pressure pulses with amplitudes P₂,comprising:explosively releasing within said body of water a charge of apressurized gas at a time t=0, where said gas has an energy Ea, so as toproduce a powerful primary pulse with an amplitude P_(o), and a cavity,said cavity tending to oscillate at a period T; injecting a secondcharge of a pressurized gas having an energy E_(b) within said regionbeginning at a time t₁ ; controlling the duration of said injection tohave a maximum value substantially equal to about (k-1/3) (E_(b) /E_(a))times the period T, where k equals the ratio of the specific heats ofthe injected gas; and adjusting the time t₁ and duration of saidinjection such that the P₂ /P_(o) ratio maintains a selected range. 19.The method of claim 18 wherein the selected range is defined as beingbetween 0-15%.
 20. A method for generating within a body of water animpulsive acoustic signal having a primary pressure pulse with anamplitude P_(o) and tending to form secondary pressure pulses, withamplitudes P₂, comprising:explosively releasing within said body ofwater a first charge of a highly pressurized gas at a time t=0, wheresaid gas has an energy Ea, so as to produce a powerful primary pressurepulse P_(o) and a cavity, where said cavity tends to oscillate about aperiod T; injecting a second charge of pressurized gas having an energyE_(b) within said region beginning at a time t₁ substantially between0.05 T and 0.5 T; and adjusting the duration of said injection such thatthe ratio of the amplitudes of the secondary pulse to the ratio of theprimary pulse maintains a range of 0-15%.
 21. A method of marine seismicexploration, comprising:(a) explosively releasing within a body of waterat a time t=0 a first charge of a pressurized gas having a pressureP_(a) and a volume V_(a), so as to produce in said water a powerfulprimary pressure pulse and a low pressure region tending to oscillate ata period T, said oscillations generating in said body of water a seriesof secondary pressure pulses; (b) injecting within said region a secondcharge of a highly pressurized gas having a volume V_(b) and a pressureP_(b), said injection of said second charge beginning substantiallybetween 0.05 T and 0.5 T; (c) controlling the duration of said injectionto have a maximum value substantially equal to about (k-1/3) (E_(b)/E_(a)) times the period T, where k equals the ratio of the specificheats of the gas of the second charge; and (d) adjusting the pressureP_(b) of said injection so that the ratio of the amplitude of thesecondary pulse to the amplitude of the primary pulse achieves a rangeof 0-15%.
 22. An explosive-type, substantially bubble-free acousticapparatus for use in a body of water, said apparatus comprising:a signalgenerator; a bubble suppressor; said generator and said suppressor eachhaving a chamber for receiving a charge of highly pressurized gas, adischarge port into the water, a movable shuttle valve for controllingsaid discharge port, and a solenoid-operated valve for controlling theactuation of said shuttle valve; each shuttle valve having a main pistonfor closing and opening the discharge port and for allowing the chamberto communicate directly with the outside medium, a control piston forcontrolling the movements of said main pistons, and a hollow shafthaving an axial bore for maintaining said pistons in spaced relation;said generator's chamber periodically receiving, in use, a first chargeof highly pressurized gas, and said suppressor's chamber, in use,periodically receiving a second charge of highly pressurized gas; saidgenerator's shuttle valve being adapted to explosively release, at atime t=0, pressurized gas (k-1 E_(b) /3 E_(a)) times the period T, wherek equals the ratio of the specific heats of the injected gas.
 23. Theacoustic apparatus of claim 22 further including means to tune theduration of the gas release from the bubble suppressor.
 24. The acousticapparatus of claim 23 wherein the means to tune the duration of the gasrelease includes at least one adjustable orifice.
 25. The acousticapparatus according to claim 22, wherein said bubble suppressor'sdischarge port is encompassed by said bubble.
 26. The acoustic apparatusof claim 22, wherein said signal generator and said bubble suppressorare arranged such that the major axis of both the generator and thesuppressor are in substantial coaxial alignment with each other.
 27. Theapparatus according to claim 22 wherein a deflector is coupled to saidsuppressor for deflecting the gas released from said suppressor'schamber and through said suppressor's discharge port into said bubble.28. The apparatus of claim 26, whereinsaid bubble has four lobes; andsaid deflector has a chamber with four outlets for deflecting the gasreleased from said suppressor's discharge ports, and for directing thedeflected gas toward respective lobes.
 29. The apparatus according toclaim 22, further comprising:a casing defining therein said signalgenerator's chamber and said suppressor's chamber.
 30. The apparatusaccording to claim 26, anda casing defining therein said signalgenerator's chamber and said suppressor's chamber.
 31. A method forgenerating within a body of water an acoustic signal,comprising:releasing within said body of water at a time t=0 a firstcharge of a highly pressurized gas having an energy E_(a) so as toproduce in said water a powerful primary pressure pulse and a very lowpressure region tending to reach a maximum volume Vm and to oscillate ata period T and to generate secondary pressure pulses; injecting withinsaid region a second charge of a pressurized gas having an energy E_(b)at least equal to (1/2 E_(a) /k-1), where k equal to the ratio of thespecific heats of the injected gas where said injection increases theinternal pressure of the low pressure region up to a maximum valuesubstantially equal to the surrounding hydrostatic pressure, where saidhydrostatic pressure is reached when the work performed by the injectedgas is substantially equal to the work done by the hydrostatic pressurebeyond the maximum volume V_(m).
 32. A method of suppressing secondarypulses in the generation of a seismic signal by injecting a charge ofgas into a body of water, comprising:injecting a first charge of a gasat a pressure P_(a) and a volume V_(a) into a body of water at a depthand in a quantity sufficient to form a first bubble resulting in aseismic signal which will tend to begin to form at least one secondarysignal at a time T following such injection; injecting a second chargeof gas into the bubble beginning at a time between 0.05 T and 0.5 T saidcharge of gas having a pressure P_(b) and a value V_(b) such that thepressure in the bubble formed by injecting the first and second chargesequals the hydrostatic pressure when the bubble volume becomesstationary; and adjusting the duration of said injection to have amaximum value substantially equal to about (k-1 P_(b) V_(b) /3 P_(a)V_(b)) times the period T, where k equals the ratio of the specificheats of the injected gas.
 33. A method of suppressing secondary pulsesin the generation of a seismic signal by injecting a charge of gas in abody of water, comprising:injecting a first charge of gas having a firstenergy E_(a) into a body of water at a depth and in a quantitysufficient to form a first bubble expanding toward a maximum volumeV_(m) and resulting in a seismic signal which will tend to form at leastone secondary pulse; injecting a second charge of gas having an energyE_(b) into the bubble at a beginning time substantially between 0.05 Tand 0.5 T and at a rate such that the energy injected into the bubble atthe time when the bubble establishes its volume V_(m), is approximatelyequal to 1.25 times the energy injected in the first charge; and meansfor injecting within said region a second charge of a pressurized gas,said second charge having an energy Eb sufficient to substantiallyestablish hydrostatic pressure within the region; and means foradjusting the duration of said injection such that said duration has amaximum value substantially equal to the product of the value (k-1)/3(E_(b) /E_(a)) and the period T, where k equals the ratio of thespecific heats of the gas of the second charge.